A wide variety of electronic devices rely on digital-to-analog converters (DACs), which convert a digital signal defined by a number of bits to an analog signal. DACs can be classified into two different categories: Nyquist rate DACs and oversampling DACs. Usually, Nyquist-rate DACs can produce higher frequency analog outputs, but use more complicated hardware. On the other hand, oversampling DACs have simpler hardware, but output frequencies of the generated analog signal are lower. One advantage of oversampling DACs is that they have a better signal-to-noise ratio, compared to Nyquist-rate DACs. High speed DACs have many applications in direct digital synthesis and software defined radio. Nyquist rate DACs typically convert digital data into analog signals by switching and adding the digital data. By toggling the switches according to input data, current or voltage is modulated. Conventionally, DACs have been made based on digital circuits, such as decoders or flip-flops for switching. Switches used for DACs are usually made with metal-oxide semiconductor field-effect transistors (MOSFETs) or bipolar junction transistors (BJTs). Use of both types of switches to support high speed operation is limited due to the severe switching speed limitations inherent in their intrinsic operating characteristics, such as carrier recombination time or transit delay. Generally, current mode switching is preferred for high speed DACs. However, due to the limited speeds of the MOSFETs and BJTs, it is difficult to increase the operating speed of the switches even using current mode switching.
In some DAC designs, the digital input data received at the DAC toggles switches, and the resulting signals are added to construct analog signal representations. Using this method, as the operating speed of the DAC becomes very high, the line length between components cannot be ignored. Generally, impedance matching between the digital data source and the sampler is poor. When the phase delay along the interconnection line is not negligible, the wave reflection should be small to avoid signal shape corruption. Unfortunately, digital data contains many harmonics to maintain their shape, and impedance matching used for reducing the wave reflection should be ultra-broadband in response. It is not easy, however, to construct ultra-broadband impedance matching circuits. A possible solution to the impedance mismatching problem is to insert buffer amplifiers whose input and output impedances are matched to the line impedances. Unfortunately, this is difficult when the input frequency of the digital signal is very high, because the bandwidth of the buffer amplifiers should cover a range from DC to several harmonic frequencies higher. Thus, what is needed is a DAC that supports high speed operation while overcoming these limitations. What is further needed is a DAC that is scalable to support the conversion of different numbers of bits and is scalable to support different frequencies.